The disclosed invention relates in general to transmission line connectors and more particularly to a center conductor contact that significantly reduces unwanted resonances and changes in the transmission and reflection characteristics. In the reference numerals utilized in the Figures, the first digit of a reference numeral indicates the first Figure in which the element associated with that reference numeral is discussed. In FIGS. 1 and 2 are illustrated an existing transmission line connection that utilizes a male connector and a female connector with a hooded female contact (i.e., the female contact is encircled by a cylindrical shell). The male connector contains an outer conductor contact 11 and a center conductor contact 12. The female connector includes an outer conductor contact 13 that butts up against outer conductor contact 11 or the male connector.
The center conductor contact of the female connector has a cylindrical shell 14 inside of which is fitted a collette 15 having a set of tines 16. The tines are bent somewhat in toward one another so that they are more closely spaced at a first end 17 of the tines than they are spaced at their bases located at a second end 18 of the tines. Center conductor contact 12 has a reduced diameter center pin 19 that is inserted into collette 15 when the male and female connectors are mated. Contact between collette 15 and center pin 19 is at a point 110 located substantially at the first end 17 of collette 15.
Unfortunately, this connector can exhibit resonances in within the useful bandwidth of the connector and therefore degrade performance of these connectors. Because the tines are not parallel, the diameter of collette 15 varies in the region of contact to pin 19 and thereby functions as a tapered transformer. When such a connector is used to connect a precision standard impedance to a transmission line, the transformer action makes the impedance seen by the transmission line different from the actual value of the precision standard's impedance. In addition, the difference between the diameter of the collette at end 17 and at end 18 is dependent on the diameter of the center pin 19 inserted into the collette. Therefore, the amount by which a precision standard load is transformed is dependent of the diameter of the center pin that is inserted into the female connector. This introduces additional variation in the characteristics of the connector. Thus, such a connector is not suitable for use in coupling a precision standard load to a transmission line. Both of these effects are accentuated in female connectors in which a hoodless center conductor contact is used.
High frequency currents are primarily surface currents so that over most of the useful bandwidth of the connectors, current flows on the inner surface of the outer conductor contact and on the outer surface of the inner conductor contact. At the interface between the male and female connectors, the current from the inner conductor of the transmission line flows along the outside of contact 12, inward along the surface of a shoulder 111, along the outer surface of center pin 19 at which point some of the current enters shell 14 and flows back to end 18 of the collette, back along the inner surface of shell 14, outward along an end 112 of the shell and then along the outer surface of shell 14. The remainder of the current from contact 12 flows directly to an end 112 of shell 14 via the capacitance between shoulder 111 and end 112. Unfortunately, the gap 113 between the tines 16 and the shell 14 is small enough that particulates can move the point of contact between the tines and the shell away from end 18 of the tines. This produces an undesired variability in the amount of impedance encountered by the inner conductor current.
In general, the characteristic impedance Z.sub.c of a transmission line is proportional to 1n (b/a) times the square root of u/e where a is the outer diameter of the inner conductor, b is the inner diameter of the outer conductor, u is the magnetic permeability of the medium between the inner and outer conductors, and e is the electric permittivity of the material between the inner and outer conductors. Reflections result at points at which the characteristic impedance changes abruptly. Therefore, to avoid unwanted reflections and resonances, the characteristic impedance should be constant along the transmission line, including at connectors between successive segments of the transmission line. Thus, in an air filled transmission line, ideal transmission would result for the inner diameter of the outer conductor and the outer diameter of the inner conductor both being constant along the length of the transmission line, including at connectors. Unfortunately, this is not always possible. For example, in an air filled transmission line, the inner conductor is kept centered inside of the outer conductor by a set of plastic dielectric beads that are spaced along the transmission line. In order to compensate for the step change in u/e at a bead, the ratio b/a is changed at the bead to hold Z.sub.c constant.
Likewise, in a connector, to avoid reflections, it is necessary to avoid sudden changes in Z.sub.c. However, in the connector of FIG. 1, gap 114 introduces a sudden change in Z.sub.c that produces unwanted reflections. Gap 114 arises because shoulder 111 is intentionally recessed relative to end 115 of outer conductor contact 11. If shoulder 111 should project beyond end 115, then center conductor contact 12 would crush the center conductor contact of the female connector when the male and female connectors are mated. To avoid this, shoulder 111 is intentionally recessed a distance on the order of the sum of the tolerances of the length of the center and outer conductor contacts. This assures that shoulder 111 does not project beyond end 115 of outer conductor contact 11.
In accordance with the illustrated preferred embodiment of the disclosed invention, a connector is presented that enables the length of the center conductor contact to be adjusted to make shoulder 111 and end 112 of shell 14 each be flush with end 115. As a result of this, gap 114 is substantially eliminated, thereby substantially eliminating reflections that are caused by that gap. The female connector has an unslotted cylindrical outer shell within which is a collette that makes contact with the center pin of a male connector. At a first end of the cylindrical shell is an opening through which the center pin of the male connector is to be inserted. At this opening, the wall of the cylindrical shell slopes inward at an angle B (measured relative to the axis A of the cylindrical shell). Similarly, at a first end of the collette, the collette tines angle inward at an angle C that is less than B and is preferably at least five degress less than B.
A spring is formed on a second end of the collette opposite to the first end. The spring fits against a shoulder of the shell and pushes the ends of the tines against the sloping inner surface of the shell to produce a wiping contact at each tine. A set of threads inside the collette enable a thin threaded shaft to be inserted through the opening in the shell and screwed into these threads of the collette. By pulling on the shaft, the collette can be removed from the shell to enable the cavity within the shell to be cleaned or to enable the collette to be replaced.